Such an electrical insulator is known from the patent U.S. Pat. No. 4,802,731. It comprises a pair of endpieces made of metal alloy, in particular an aluminum-based alloy, fixed to corresponding ends of a support that provides mechanical strength and that has an outside surface whose shape is generally that of a cylinder, a truncated cone, or some other body of revolution. The endpieces serve as anchoring elements for the insulator. The support is made of an electrically insulating composite material, e.g. a winding of organic or inorganic fibers, such as glass fiber, embedded in a settable synthetic resin, such as an epoxy resin.
The insulator also includes insulation made of elastomer material covering in part the outside surface of the support and in part the outside surface of each endpiece so as to form annular fins which serve to increase the creepage distance of the insulator. Because of their greater strength and smaller weight, insulators of the above type are progressively replacing traditional insulators made of porcelain.
Each end of the support of the insulator is inserted in a nozzle-shaped housing provided in a corresponding endpiece of the insulator, the endpiece being fixed to the support by adhesive and by an interference fit, for example. Because the endpiece is assembled in this way to the support, the outside diameter of the endpiece at the opening leading to the housing in which the end of the support is inserted is necessarily larger than the outside diameter of the support level with the opening.
It is essential to avoid trapping air inclusions between the outside surface of the support and the elastomer insulation, or between the portion of the outside surface of each endpiece and said insulation since such air inclusions give rise to incandescent electrical discharges. Air inclusions also give rise to the elastomer material insulation becoming deformed during vulcanization. Unfortunately, the elastomer insulation is generally applied to the outside surface of the support and of the endpieces by the technique of helically winding a shaped strip of elastomer material. As a result, if the difference in diameter between the endpiece and the support at the opening of the housing in the endpiece is very great, it would appear not to be possible to apply the technique of winding a shaped strip of elastomer material to build up the insulation while also avoiding any air inclusions at the opening of the housing. According to the above-mentioned document U.S. Pat. No. 4,802,731, and as can be seen in FIG. 5 of that document, a tapering transition surface is provided at each endpiece so as to obtain a progressive reduction in diameter going from the endpiece towards the support. This tapering surface thus joins the outside surface of the endpiece to the outside surface of the support. However, the tapering surface is defined by a tapering chamfer that forms part of the endpiece, with the chamfer being obtained by machining the endpiece.
The operation of machining an endpiece to obtain a tapering chamfer with adequate surface state for receiving the shaped strip constituting the installation is expensive because it must be accurate. In addition, because the chamfered end of each endpiece is angular, when the electrical insulator is in use or when it is subjected to electric shocks due to lightning, the end serves as a support for setting up electric fields that are particularly intense and this harms the insulating qualities of the insulator and increases the risk of incandescent discharges.